RE DEFAULT
The default presentation is what most users get when they go to the site
for the first time with a browser that is set up in the form it comes
when you install it. (e.g. no user preferences set).
What about window size? With dynamic reformatting, there may be
relationships that are visually evident in one layout but not visually
evident in another, but may still be needed to understand the content.
Do all user agents present content the same way? Must a header be
visually distinct from a paragraph in HTML, for instance?
I still think "default presentation" is scary ground when we are trying
to identify what is needed for alternative representations.
If the author presents the relationships to most users (even if not all
need them all) then the relationships should be provided for all. They
are information - therefore they should be perceivable even if view not
using the expected/standard/default means for viewing.
So if an image has any visual substructure, e.g., a picture of a
bicycle, with wheels and a frame, that substructure would need to be
represented programmatically, even if the bicycle is just being used to
represent sports in general?